This invention relates to an air cooling equipment for use in electronic systems such as communications systems and information processing systems, and more particularly to an air cooling system for electronic components mounted on printed circuit wiring boards installed in those systems.
In general, active elements such as transistors mounted on the printed circuit wiring board tend to generate heat proportional to the dissipated electric power. The heat produced has an adverse effect on characteristics of the active elements and, if too great, can lead to the destruction of those active elements. For this reason, a strict temperature restriction is imposed on these electronic components to ensure reliability. This restriction is easily met in circuits using only a few active elements; however, an increase in the number of active elements is accompanied with the increase of the electric power dissipation and the quantity of heat produced which must be dissipated. This is typically accomplished by means of a cooling equipment for effectively cooling the electronic components so as to maintain the temperature of the electronic components below their maximum operating temperature.
One attempt for the practical use of such a cooling equipment is disclosed in a liquid cooling system in Japanese Patent Application Disclosure No. 8776/1977 (corresponding to U.S. patent application of Edward A. Wilson et al, Ser. No. 592,578, now U.S. Pat. No. 4,072,188, filed Feb. 7, 1975, and assigned to HONEYWELL INFORMATION SYSTEMS, INC.). However, a complicated mechanism for liquid flow is unavoidable in such a system, which consequently leads to a bulky system. Also, as the system becomes bulky, defective parts tend to occur in the system. As a result, in the event that a hole is found in a liquid passage, liquid leaks therethrough with the result that the intended cooling function cannot be achieved. Another attempt for that purpose is disclosed in a gas (air) cooling system shown in FIG. 194(A) on page 327 of Heat Transfer In Micro-Electronic Equipment (A Practical Guide) by J. H. Seely and R. C. Chu, published by Marcel Dekker, Inc., New York in 1972. This system supplies air as a coolant upwards from a blower mounted at the lower end of a support so as to cool electronic components provided on a plurality of component cards. The temperature T of the heat-generating electronic components, when such an air cooling system is used, is given as follows: EQU T=Ta+dTa+dTb+dTc (1)
wherein Ta represents the temperature of a room with a system provided with the electronic components, dTa represents a temperature rise at the inlet of a duct relative to the room temperature, dTb represents a temperature rise measured around those electronic components as compared with the temperature at the inlet of the duct, and dTc represents a temperature difference attributable to the thermal resistance prevailing from ambient air surrounding the electronic components to the components. In this respect, dTa in the second term of the right-hand side of equation (1) is negligible because the air at the room temperature is directly used for the purpose of cooling. Accordingly, the considerations in the design of a cooling system should be paid to a decrease in dTb and dTc in the third and fourth terms of the same side of equation (1), respectively. In this case, dTc is inversely proportional to an air velocity, but if the air velocity exceeds a given level, then the tendency of a decrease in dTc decreases markedly. As a result, an important factor to be considered in cooling the heat-generating electronic components is dTb. The factor dTb is inversely proportional to the flow rate of air supplied as the coolant and decreases with an increase in the flow rate of the coolant air. Meanwhile, the air flow rate varies due to the system resistance increasing in proportion to the length of air passage, thus failing to provide air flow rate of more than a given level. For this reason, in order to obtain the air flow rate required for fully cooling the electronic components, a bulky air blower is needed resulting in a high level of noise.